1. Field of the Invention
The present invention relates to color matching systems, or to systems for translating data representing a color from a source color graphics device to data representing a color in a destination color graphics device.
2. Description of Related Art
A large number of color graphic devices, such as CRT display systems, printers, plotters, film recorders, programs using color, and others are now available to computer users. This explosion in color technology complicates the problem of matching color amongst the various devices. That is, colors created for a CRT screen are not typically reproduced well on an ink jet printer, and likewise for other combinations of source and destination devices.
Colors have been characterized by three components; luminance, hue, and saturation or chroma. This fact has been translated into a variety of color spaces which have been calibrated based on a standard human perception of colors. A common calibrated color space is referred to as the CIE tristimulus space. Another commonly used calibrated space is called the CIE chromaticity space. The advantages of the CIE tristimulus coordinates arise from the fact that the coordinates are always positive in value, the entire range of color vision is represented, and the color mixing equations are linear. There are a wide variety of other color spaces that have standardized definitions, and could be considered a calibrated color space, such as L*u*v*, and L*a*b.
Other color spaces, such as RGB, have no standardized definition. They provide virtual devices with simple conceptual models, rather than precise standard models. These conceptual models allow artists or programmers to create colors for virtual devices independent of production equipment. These colors can then be supplied to production equipment and yield at least intuitively consistent results.
Additional background concerning the wide variety of "color spaces" is found in Billmeyer, Jr., et al., PRINCIPLES OF COLOR TECHNOLOGY, 2nd Ed., John Wiley & Sons, 1981; Taylor, et al., Tektronix HVC: A Uniform Perceptual Color System for Display Users, Tektronix Laboratories, Beaverton, Oreg., (date unknown); and Engeldrum, "Four Color Reproduction Theory for Dot Formed Imaging Systems", Journal of Imaging Technology, Vol. 12, No. 2, Apr., 1986, pp. 126-130.
Color matching is the process of determining the combination of destination system colorants that will produce a color which matches a color produced on a source device. To achieve color matching, careful balancing of the hue, chroma, and luminance constituents of the source color must be accomplished. Further, often it is not possible to produce a source color on a destination system. Thus, some adjustment to the hue, chroma, and/or luminance of the source color may be necessary to achieve a producible color.
Also, it may be important to compensate for perceptual factors in color science, such as metamerism and color constancy, by adjustments to the destination color to achieve a desired color effect. Metamerism is the effect where two samples will match under one set of conditions, but do not match under different conditions. The phenomena of color constancy arises because the human visual system constantly readjusts the color balance of our perception to maintain a constant interpretation of color as illumination changes. For instance, the achromatic or gray colors on the source and destination devices may not match because of differences in colorants and media among source and destination devices. Thus, colors that appear gray on a first device, may not appear gray on a destination device unless an adjustment is made to account for these perceptual effects.
Accordingly, in any color matching algorithm, balancing of the three color constituents must be made to achieve a perceptually pleasing result. Typically, the luminance component, also called lightness, carries the most information and should be given first priority in a matching algorithm. See Hunt, R. W. G., The Reproduction of Color in Photography, Printing and Television, 4th Ed., Fountain Press, Tolworth, England, 1987, pp. 43-44. Hue is often considered the next most important component. The chroma, also called saturation, is often considered the least important component in color matching algorithms, and is the one most likely to vary among production devices.
Prior art systems for performing color matching are typified by time consuming algorithms such as disclosed in Myers, U.S. Pat. No. 4,751,535. The Myers patent is incorporated by reference for providing an excellent discussion of the background of color matching. These algorithms have proved too slow for practical application. Also, the prior art algorithms are typically optimized for single source and destination device pairs.
Alternatively, prior art systems use very large look-up table systems which require vast storage for accurate color translation. These large table look-up systems have been too costly and inflexible for practical use, except for specialized systems. Furthermore, the large table lookup systems are limited to single source and destination combinations and cannot be easily expanded to other devices.
Accordingly, it is desirable to provide a method and an apparatus for translating a color from a source device to a color on a destination device that operates quickly, that does not require huge amounts of data storage, and can be applied independent of the source and destination devices.